The current process for producing flat rolled aluminum sheet products for markets such as automotive, rigid container, can body and can ends, involves casting ingot, scalping the ingot, homogenization of the scalped ingot, then breaking down the ingot in hot reversing mills, followed by a continuous hot mill ultimately producing a coil of aluminum alloy. The coil either self-anneals or requires batch annealing before cold rolling to final gauge. Common alloy sheet, such as that from the 1XXX and 5XXX series alloy typically used for inventory in distributor stock, is also produced by a similar process. The advantage to this ingot based process is that it is a proven technology capable of consistently delivering the combination of strength, formability, surface quality and other product specific characteristics required by various markets.
The above recited process is inherently flawed, however, from an economic point of view due to high capital costs required in such a manufacturing process. These high capital costs swell from the apparatus required to perform the various process steps such as casting, scalping, homogenizing, and hot rolling. Recovery costs associated with scalping, end cropping and excess trimming on hot mills can cause wastage of salable alloy in any particular run of up to 25%. The ingot based process also requires high inventories to be maintained by an aluminum alloy producer and/or distributor since the process is not considered a "real time" process due to process discontinuities. These discontinuities, such as the homogenization and ingot breakdown steps, can be a major cause of mechanical property inconsistencies that are introduced into the product stream from coil to coil and even within an individual coil.
Many conventional ingot based processes capable of producing quality automotive and can sheet exceed 500 million pounds of aluminum alloy product in annual capacity. In fact, such a capacity is needed since at lower capacities manufacturers suffer a higher capital cost per pound of output making the economics difficult to maintain and/or justify. This underscores the need to solve, not just the quality problems often associated with aluminum alloy production, but more importantly, the economic dilemma. The invention hereof shines since it can help to reduce capital costs and therefore, reduce the requirement for high throughput thus making implementation of this new process more economical on a per pound basis. This may allow smaller volumes of alloy production and, therefore, smaller plants at a higher cost effectiveness.
A continuous caster, either slab or roll caster, may be inherently more cost effective simply because it does not require the homogenization, scalping, and ingot break down as part of its process. For reasons discussed below, the application of continuous casting aluminum sheet has been limited to lower solute non-heat treatable aluminum alloys for non-surface critical applications. Commercial roll casters almost exclusively produce stock for processing to foil gauges. Slab casters produce re-roll for non-surface critical sheet products such as residential building products for, as examples, aluminum siding and/or down spouts, furniture tube, and/or distributor stock. Non-surface critical applications means that the ultimate consumer is not, for example, in the food or automotive businesses where surface blemishes cause the can or, automotive stock to fail customer aesthetic standards and/or specifications.
There remain, therefore, commercial problems in the can and automotive sheet markets which the continuous casters of the prior alt have yet to resolve. These problems relate to insufficient surface quality, inadequate strength and/or formability combinations coupled to the commercial realities of a capital intense business.
In a continuous cast product, surface quality is strongly influenced by the cast surface since the scalping operation is not performed. Liquation, surface segregation and other surface heterogeneities, common to continuous caster processes, remain problematic for prior technologies.
In terms of strength and formability, thermal processing of slab cast material by traditional batch process can be a handicap due to limitations in crystallographic texture control as a consequence of the absence of a homogenization step and minimal hot rolling. Additionally, solute levels are reduced because of slow cooling from the soak temperature resulting in relatively low work hardening rates. This creates difficulty in body stock for the can industry, for example, since attaining acceptable combinations of strength and earing are near well impossible to achieve.
U.S. Pat. No. 4,238,248 addresses this problem from an historical perspective as a continuous heat treatment in combination with slab casting to achieve acceptable combinations of strength and surface characteristics. Having said that, heretofore, a continuous casting process capable of meeting the surface, strength and formability requirements of automotive, and separately can body and end stock, while producing at a manufacturing scale appropriate for market demand, has not been commercially available.
U.S. Pat. No. 5,356,495 discloses a continuous caster process. This patent does not specifically discuss the problems addressed hereunder such as combinations of strength, formability, and surface quality.
U.S. Pat. No. 4,614,224 discloses a continuous casting process but also avoids discussion of the combinations of formability, strength, and surface quality.
In a more recent effort U.S. Pat. No. 5,616,189 discloses a continuous casting process, in particular a twin belt caster, that outputs 6000 series aluminum alloys for the automotive market. Again, the combinations hereof discussed are simply ignored. The above efforts indicate that continuous caster processes are difficult to implement and still be competitive with the ingot based technology, elsewise specific discussion of the problems solved in the continuous caster product, the formability, strength, and surface integrity, would be a center piece of the disclosure.
The problems remain, however, and the invention hereof is directed to solving these problems. Accordingly, the present invention is useful for the manufacture of automotive sheet, can stock sheet, and can end stock sheet with a product that has comparable formability, strength, and surface integrity to that of World Class Ingot technology. "World Class Ingot" as used hereinafter, is a standard of sheet made from ingot with use of the most developed processes by which continuously cast aluminum alloy sheet is compared. Heretofore, typical surface characteristics and properties made from continuously cast aluminum alloy sheet could not compare to the surface characteristics and properties of product made by the ingot process.